1. It's an Old Leaf. - Leaves are not forever, even on evergreen tropicals like your houseplants. Like trees losing their leaves in winter, all plants cycle through leaves eventually. Sometimes it is because a leaf was damaged in some way and sometimes it is just because the leaf is old. When a leaf has served its purpose, the plant will withhold nutrient from it, causing it to turn yellow and eventually fall off. That is just a normal part of plant growth. Trim off the yellowing leaves and you'll see plenty of young green leaves emerge.

2. Over or Under-Watering - If a plant is stressed from too little water, it may withhold energy from some of its leaves as a defense mechanism to conserve resources. The same thing also happens when a plant sits too wet for an extended period of time. Water consistently to keep a plant happy.

3. Time to Repot - If your plant has been in its container for a while, it may be aching for room to grow. Over time, as roots overwhelm the soil in a small pot, a plant is unable to take up nutrients and water properly. Repot your plant into a larger pot and watch your plant breathe a sigh of relief.

4. Lack of Light - If your plant seems like it is turning yellow on one side, especially if that side is away from its primary light source, it may not be getting enough light. If it is just one side, try rotating the plant each week so that the plant gets light all around or try a new location that is a little brighter.

5. Fertilizer - Nitrogen and iron in fertilizer help keep a plant lush and green. When a plant isn't getting those nutrients, the overall color of the plant becomes pale and yellowish. Give the plant a dose of nutrients on a regular basis to keep leaves healthy and a rich green color.